Railroad rails or tracks are most often supported by railroad ties (or rail ties) connecting the tracks together and transferring the loads applied by the locomotive and rail cars to the materials below. Rail ties are typically supported by a bed of ballast (e.g., large aggregate) that is placed over the existing ground. The aggregate serves as both a drainage layer and a load support layer.
When railroads are constructed over soft soils, or when deep embankments are required to be constructed for rail grades, the ground below the aggregate can settle or have low stiffness, resulting in too much deformation and permanent settlement of the supported aggregate, rail ties, and rails. Settlement, particularly when non-uniform, and low track modulus often results in the reduction of allowable train speeds causing unwanted economic inefficiency for rail operators and frequent maintenance. Furthermore, problems with settlement and low stiffness are often exacerbated by rainfall. The aggregate tends to “settle into” the underlying soil, forming a curved interface between the bottom of the aggregate and the top of the subgrade with the maximum settlement at or near the center of the rails and less settlement along the outward edges of the ties. Rainwater then percolates through the aggregate and is trapped by the “bathtub” of the curved interface. This water then does not drain quickly and seeps into the underlying soil further softening and weakening this material.
There are many existing methods to stabilize rail beds that have settled. Over-excavation and recompaction is a method in which the rail and ties are removed, the aggregate is removed, and the underlying soft soil is excavated to a depth sufficient to remove the soft and compressible materials. Stronger backfill is then brought in, placed, and compacted, and the rail bed is reconstructed. This method has the disadvantages of being expensive and highly disruptive to existing rail traffic.
Lime and cement stabilization methods have also been used to stabilize the soft materials. Lime and cement slurries are injected from the top or sides of the rail bed to interact with the compressible clay soils, to fill voids in the aggregate, and to add strength and stiffness to the system. These methods have the drawbacks, however, of having a relatively high cost and a relatively high rate of failure because of the difficulty of getting the materials to seep into and mix with the compressible soils.
Drains are also sometimes used to remove water from rail beds. Drains often consist of perforated plastic pipes inserted into the bedding aggregate and “daylighting” onto the side of the rail embankment. This method has the advantage that it is expedient and can be installed from the side of the operating line. However, drains clog and the method provides for a passive rather than an active solution and is not reliable for improving design track modulus.